What do we need to do to eliminate the need to water leach?

I read the response to the inquiry, ‘Should I flush my growing medium?’ in your magazine. My question is, “what do we need to do to eliminate the need to water leach”, which means matching the nutrient solution formulation and its use factors based on the water-holding capacity of the rooting medium, which no one is investigating. So, we slosh on the nutrient solution and then monitor the EC of the retained solution, and at some point water leach to remove the accumulated ion, known as ‘salts’. What a waste!

Answer

Thanks for your interesting and unusual question.

Flushing
In my answers I try to cover most options related to the question in general, rather than give just a single answer to the specific question. You are reminding me that I need to concentrate more on giving a simple unambiguous answer, before expanding into wider aspects.

My previous answer was looking specifically at media-based systems (the most widely used by both commercial growers and hobbyists). If you read my previous answer again you will find that I was making two major recommendations:

• Provided you kept the EC of your run-off under control, there should be no need to flush the medium.

• If you do need to flush you should always use nutrient solution, not water.

Discarding nutrient
One aspect that I didn’t cover last time was what to do with the run-off solution? That is, do you recycle it into the system (which is the preferable procedure) or do you discard it? Discard can be used responsibly such as on pasture or your garden, etc, but is still ‘waste’.

In this answer I’ll concentrate on how to reduce or preferably eliminate wasting nutrient solution. In contrast to your comment, there has actually been a huge amount of investigation done into nutrient uptake and the Dutch have published root zone solution guidelines since the 1970’s for a wide range of crops.

Factors influencing waste
Numerous factors influence whether your system can go without having to discard solution, or for how long before discard is needed. The most important are:

• The quality of your water supply, especially the sodium chloride content. If the level is high, especially sodium, it will build up within the root zone solution until it is high enough to damage the plants. The more sensitive your plants and the higher the level in your water, the quicker this happens. If the salt level is too high, the only practical answer is to remove it with reverse osmosis.

• How close your fertiliser formulation is to the uptake of the plant. Commercial growers get regular analyses of the run-off solution to keep it within range of the recommended root zone solution analysis.

• How much acid (or alkali) you add for pH control. Often too much emphasis is placed on tight pH control. For example, the more acid you add to reduce your root zone solution pH, the more you throw your nutrient solution out of balance.

System types
There are several different types of ‘closed’ hydroponic systems.

The media-based systems I mention above have drippers feeding containers of growing medium. In commercial operations about 1/3 of the feed volume is run-off, collected, disinfected and recycled. For nutrient management the run-off solution is periodically analysed and the feed adjusted as needed to keep the run-off nutrient balance within the guidelines.

Other closed systems include: Continuous flow channel systems (NFT – nutrient film technique); Flood and drain – usually hobby systems using beds of gravel or expanded clay; tank systems with floating rafts or fixed beds above. Commercial growers also nutritionally manage these by periodic solution analysis. However disinfection is more difficult because of the larger solution volumes involved and the use of oxidising chemicals runs the risk of also damaging plant roots.

Options for hobby growers
Chemical analysis is too expensive for most hobby growers, however if you discard solution regularly from your system, there are things you can do to reduce this wastage by keeping your solution longer before disposing of it.

Step one is to know what’s in your water supply. The EC of the water gives you a good guide. For water of EC 0.2 mS/cm (2 CF) or lower, this is of good quality and will enable you to go an extended period between discards. If you have rain water, that’s great. As the water EC gets higher, then the period between discard is reduced. Once you get above an EC of 0.5 mS/cm, if it is mainly sodium chloride, then recirculating becomes doubtful, and you may need to consider using reverse osmosis. The exception here is if you have ‘hard’ water, in which case the calcium and magnesium are used as nutrients.

How fast your solution gets out of balance also depends upon the growth rate of the plants. That is, when the plants are small, or in the colder seasons, the amount of solution uptake will be less. In practical terms, if you believe that you have to discard solution, this should be done less often in winter than in summer.

Water holding capacity
The water holding capacity of the medium within a system and especially its volume are factors that have a major influence on your hydroponic management. The smaller the volume of root zone solution per plant that you have in your system, then the faster the properties you are trying to control will change. That is, if you save money by using a small volume per plant the management of pH, EC, water content, solution temperature, etc, all become much more difficult. This particularly applies to nutrient balance, hence the higher the volume of solution per plant in your system, the longer between discards.

More information on nutrient management
I gave a series of four columns giving more detail on nutrient management in Practical Hydroponics & Greenhouses Issues 139 through 142 (January through April 2014). Ω